In Lake Ontario, thermal stratification normally occurs in late May or early June and lasts through October. During stratification period, the lake develops an upper layer of a relatively uniform warm water (epilimnion) , that is characterized by increased circulation and sometimes turbulence. The thermocline (a thin layer of maximum temperature gradient) separates the upper warm waters from the colder and denser bottom layer of the lake, the hypolimnion. The setup position of the thermocline depends of the wind regimes, with higher position during calmer springs. Thermoclines favor the formation of internal gravity waves generated by atmospheric pressure fluctuations, wind, non-linear wave-wave interaction, buoyancy fluxes, interaction of basin-scale motion with bottom topography or instabilities in the mean currents [7].
Even if the difference between the top and the bottom of the thermocline is of only a few degrees, it is sufficient to prevent circulation of water between the hypolimnion and the epilimnion [9]. This density barrier separates the circulation of the hypolimnion from that of the epilimnion [8, 11].
Wind-induced transport of heat is usually of greater importance than direct solar heating in Lake Ontario where the light is rapidly attenuated with increasing depth [9]. This is particularly true on a shore stretch between Toronto and Kingston where a steep underwater escarpment enhances the effect of upwelling events that are frequent on the north shore of Lake Ontario.
Wind stress applied to a stratified lake forms a turbulent mixed epilimnion. Turbulence distributes the wind momentum and heat through the depth of the epilimnion which moves downwind [10]. In the event of prolonged wind stress the epilimnion will gradually deepen due to enhanced turbulent kinetic energy (TKE) caused by breaking waves, coupling of pressure fluctuations between wind and water, convective penetration, velocity shear in drift currents or Langmuir cells. During the season changes, in spring when the surface water is warmer, or in the fall when it is cooler, these processes facilitate the penetration through the density barrier [12]. In the fall, this progressive erosion of the thermocline eventually leads to the fall overturn eventually barotropic lake conditions [9]. Winter water temperatures vary only slightly between 0OC and 4OC [14].